During the development of the Joint Direct Attack Munition (JDAM) a need arose to precisely determine when the munition's fusing mechanism under test generated a firing command to trigger the warhead of the weapon. Since the tested weapons were outfitted with inert warheads, a non-explosive method was required to demonstrate fuze functionality.
JDAM weapons are designed to be carried aloft while attached to a store point of an aircraft or in the aircraft's bomb hold. Each JDAM includes an unguided (i.e. “dumb”) bomb and a kit attached thereto that includes a Global Positioning System (GPS) based guidance subsystem. The guidance subsystem includes adjustable fins, actuators, a processor, and other associated components that convert the bomb to a guided (i.e. “smart”) weapon. Service personnel typically load the JDAMs on to the aircraft hours before the intended use of the weapon. At some time prior to release, the GPS coordinates of the intended target are loaded into the guidance system. The aircraft then flies to the vicinity of the target and releases the weapon at a location that is pre-calculated to allow the weapon to fall toward the target. While the JDAM is falling, the guidance system adjusts the trajectory of the weapon to cause it to strike the target with little, or no, positioning error. At a pre-selected altitude nearly coincident with the weapon's impact, the fuze receives a signal from an on-board DSU-33 (radar altimeter) that indicates that the desired height above the ground has been achieved and the fuze under test initiates the fire signal to a “simulated” explosive charge. The fuze initiates upon receiving the command from the DSU-33 and, if explosives are included in the warhead, triggers the explosive material. Because the bomb typically falls at a speed approaching Mach 1, the pre-selected altitude allows the explosion to propagate through the explosive material in such a manner as to cause the weapon to explode within a short distance from the target. Thus, the JDAM kit allows the user to convert an unguided weapon to a low cost guided weapon with precision strike capabilities. Such precision strike weapons guidance subsystems are available from the Boeing Company of Chicago, Ill.
To keep unit costs low, and to avoid undesirable modifications of the associated aircraft (e.g. the addition of a power umbilical), the JDAM is designed to be self sufficient, particularly with regard to power. Thus, each JDAM includes a 28-volt thermal battery to power the guidance subsystem. Because it is likely that the JDAMs will be stored on the aircraft for many hours prior to their use, the power supplied by the thermal battery must be reserved for the guidance system.
Nonetheless, it is still necessary to know within about 1 foot of altitude when the fuze commands the detonation to determine the reliability of the fuze, particularly with regard to the timing of the explosion vis-a-vis the approach of the weapon to the target. Thus, a telemetry system is typically added to the test JDAM to transmit the weapon fuze command, engineering information, and other data to the test data system. Unfortunately, as the JDAM nears the ground, the telemetry signal reflects off of the ground and structures thereabout. These reflections interfere with the original signal and therefore cause loss of the transmitted data. The transmitted fuze command suffers disproportionately from this interference because it typically occurs within a few feet of the ground where such multi-path interference is most severe. Thus, a need exists to reliably and precisely determine when and where the fuze command occurred even with the presence of multi-path interference with the telemetry signal.